1. Field of the Invention
The present invention relates to an electromagnetic clutch used for connecting and disconnecting power transmitting from a power source to a compressor and, in particular, to an electromagnetic clutch in which a friction member is provided on a side end surface of a rotor onto which an armature plate comes in contact.
2. Description of Related Art
As illustrated in FIGS. 10A and 10B, a general electromagnetic clutch used in a compressor of a vehicular air conditioner includes a rotor 4 that is formed of a magnetic material, is fitted onto an outer periphery of a cylindrical portion 16 through a bearing 14 to be rotatable in which a driving shaft 20 of a compressor 2 is journaled, and is rotated by receiving a rotational force from a rotation driving source; an armature plate 5 that is formed of a magnetic material, is connected to the driving shaft 20 to be relatively non-rotatable, and is disposed to face a side end surface of the rotor 4 across a slight gap; an excitation coil 3 that is accommodated in an inside of the rotor 4, forms a magnetic circuit through the rotor 4 and the armature plate 5 by being energized, and generates an attracting force that electromagnetically attracts the armature plate 5 onto the rotor 4; an outer magnetic blocking portion 21 provided on the side end surface of the rotor 4, that is configured with an annular shape centered on the driving shaft 20 or a plurality of arc shapes disposed on a virtual circle centered on the driving shaft 20; an inner magnetic blocking portion 22 provided on the end surface of the rotor 4, that is configured with an annular shape centered on the driving shaft 20 or a plurality of arc shapes disposed on a virtual circle centered on the driving shaft 20, which is provided inside further than the outer magnetic blocking portion 21 in a radial direction; and an intermediate magnetic blocking portion 23 provided in the armature plate 5, that is configured with an annular shape centered on the driving shaft 20 or a plurality of arc shapes disposed on a virtual circle centered on the driving shaft 20, which is formed in a position facing a position between the outer magnetic blocking portion 21 and the inner magnetic blocking portion 22 in the radial direction.
A temperature fuse 15 which blocks electric connection to the excitation coil 3 when exceeding a predetermined temperature is provided on the excitation coil 3 side from the side end surface of the rotor 4. If the compressor 2 becomes unrotatable for some reason, slip occurs between the rotor 4 that is rotated by receiving the rotational driving force from the driving source and the armature plate 5 that is connected to the driving shaft 20 of the compressor 2 that is unrotatable. The temperature fuse 15 is fused by friction heat caused by the slip and a connection of the clutch is cut (electromagnetic attracting force that attracts the armature plate 5 onto the rotor 4 is removed) by blocking electric connection to the excitation coil 3.
In addition, in order to reduce occurrence of unpleasant noise when the armature plate 5 hits on the rotor 4, a configuration is proposed in which an annular groove 24 is formed on the side end surface of the rotor 4 facing the armature plate 5 so as to include the outer magnetic blocking portion 21, and a ring-shaped friction member 27 formed of a non-magnetic material such as thermosetting resin is mounted on the annular groove 24 in a state of being slightly protruded from the side end surface 4b of the rotor 4. In this configuration, when the armature plate 5 is attracted toward the rotor 4 side by the electromagnetic force of the excitation coil 3, the armature plate 5 comes in contact with the friction member 27 before coming into contact with the side end surface of the rotor 4 (JP 2008-014444 A).
The friction member 27 is disposed to cover the outer magnetic blocking portion 21 for ensuring a magnetic path and is slightly protruded from the side end surface of the rotor 4. Since the friction member 27 and the armature plate 5 have an elasticity in themselves, when the armature plate 5 is attracted to the rotor 4, the armature plate 5 comes into contact not only with the friction member 27 but also with the side end surface 4b of the rotor 4 in a region close to an inner periphery than the friction member 27. As described above, when the rotor 4 and the armature plate 5 are engaged, both of the friction member 27 and the inward region of the side end surface than the friction member 27 come contact with the armature plate 5, thereby performing stable transmission of torque.
However, if the compressor becomes unable to rotate and slip occurs between the rotor 4 and the armature plate 5, sliding occurs at a portion close to the inner periphery of the rotor 4 and the armature plate 5, which are similar magnetic metal materials, and hence adhesion is likely to occur. If the adhesion between similar metal materials occurs at a region close to an outer periphery of which a peripheral speed is fast, a relative speed difference is large, generated friction heat is also large, and thereby it does not lead to sticking. However, if the adhesion occurs at a region close to the inner periphery of which the peripheral speed is slow, since both a difference in a relative speed and occurring friction heat are small, hardening is likely to occur between melted metals and a sticking phenomenon of a sliding surface is caused before the temperature fuse 15 is operated. As a result, there is a concern that the engagement of the clutch is not disengaged without fusing of the temperature fuse 15.